A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a “mask” or a “reticle,” may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (i.e., resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called “steppers,” in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called “scanners,” in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is necessary to measure parameters of the patterned substrate such as, for example, the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known—spectroscopic scatterometers and angularly-resolved scatterometers. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly-resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
Scatterometers may be used to measure several different aspects of lithographic apparatuses, including their substrate orientation and exposure efficacy. Two important parameters of a lithographic apparatus (and specifically, of the exposure action that the lithographic apparatus carries out) that may also be measured by scatterometers are focus and dose. A lithographic apparatus has an exposure apparatus that includes a radiation source and a projection system as mentioned below. The radiation source provides a beam of radiation and the projection system focuses the beam of radiation and applies a pattern to the beam to create a patterned beam of radiation that strikes the resist on the substrate surface.
The dose of radiation that is projected onto a substrate in order to expose it is controlled by various parts of the exposure apparatus. It is mostly the projection system of the lithographic apparatus that is responsible for the focus of the radiation onto the correct portions of the substrate. It is important that the focusing of the image of the pattern in the patterned radiation occurs at the surface of the substrate where the exposure occurs. This is so that the sharpest (i.e., most focused) image will occur on the surface of the substrate and the sharpest pattern possible may be exposed thereon. This enables smaller product patterns to be printed.
The focus and dose of the radiation directly affect various parameters of the patterns or structures that are exposed on the substrate. Parameters that can be measured using a scatterometer are physical properties of structures within the patterns that have been printed onto a substrate. These parameters may include the critical dimension (CD) or side wall angle (SWA). The critical dimension is effectively the mean width of a structure such as a bar (or a space, dot or hole, depending on what the measured structures are that are in the printed pattern). The side wall angle is the angle between the surface of the substrate and the rising (or falling) portion of the structure.
In addition, mask shape corrections (focus corrections for bends in a mask) can be applied if scribe lane structures are used with a product mask for focus measurements.
Focus and dose may be determined simultaneously by scatterometry (or scanning electron microscopy) from a one-dimensional structure in the mask pattern (which gives rise to a one-dimensional pattern on the substrate, from which measurements are taken). A single structure can be used as long as that structure, when exposed and processed, has a unique combination of critical dimension and side wall angle measurements for each point in a focus energy matrix (FEM). If these unique combinations of critical dimension and side wall angle are available, the focus and dose values can be uniquely determined from these measurements.
However, there is a problem with this use of one-dimensional structures. There are generally several combinations of focus and dose that result in similar critical dimension and side wall angle measurements. This means that focus and dose cannot be determined uniquely by measuring a single one-dimensional structure. It has been considered to use more than one structure in separate adjacent markers to resolve this ambiguity. However, having a plurality of markers incorporating different structures has disadvantages, including occupying potentially valuable space on the substrate surface.
A focus offset or error during the exposure of a pattern on a target portion of a substrate can be measured only indirectly. For example, to measure the side wall angle, the whole profile of the pattern on the target is reconstructed. The focus is then derived after calibrating a model that describes, for example, side wall angle and critical dimension as a function of focus and dose. This technique is known as focus-dose separation.
The scatterometry signal sensitivity towards variations in side wall angle (and CD) gives rise to derivations of focus (and dose) values. However, the sensitivity of the scatterometry signal (i.e., the reflected radiation that contains information regarding the surface from which the radiation was reflected) is affected by the thickness of the radiation sensitive material (i.e., resist) on the substrate surface. In fact, the sensitivity of the scatterometry signal may scale with the inverse square of the resist thickness.
A decreased sensitivity may lead to the following unwanted effects: although the sensitivity level may decrease, the noise level does not decrease and as a result, the signal-to-noise ratio decreases and the side wall angle reproducibility may deteriorate accordingly; as modeling errors remain the same, this may lead to increased systematic accuracy errors in the side wall angle measurements; and, resist heights variations, or other variations in a stack that is part of the modeled pattern, may give rise to an undesired impact on the side wall angle measurements (also known as cross-talk).
The above-mentioned unwanted effects may have a direct impact on the focus values derived from the side wall angle.